President Barack Obama has said the US government will do everything it can to help communities recover from devastating tornadoes that have killed at least 297 people in the south-east.
At least 204 people died in Alabama, which Mr Obama will visit on Friday.
The president hailed rescue workers and said he stood with every American affected by the "catastrophic" storms.
A state of emergency has been declared in seven states, and federal aid money is being sent to Alabama.
Deaths and widespread devastation are also reported in Tennessee, Mississippi, Georgia and Virginia.
Speaking at a news conference at the White House, Mr Obama said: "The loss of life has been heartbreaking, especially in Alabama.
"In a matter of hours, these deadly tornadoes, some of the worst we have seen in decades, took mothers and fathers, sons and daughters, friends and neighbours, even entire communities.
"In many places the damage to homes and businesses is nothing short of catastrophic," he said.
The president said he would travel to Alabama to meet those leading the emergency response and families who are "reeling from the disaster".
"I want every American who has been affected by this disaster to know that the federal government will do everything we can to help you recover, and we will stand with you as you rebuild," he said.
The US National Weather Service has reports of nearly 300 tornadoes since the storms began on Friday, more than 150 of them on Wednesday alone.
The National Weather Service's Storm Prediction Center has said some of the tornadoes on Wednesday may have been more than a mile (1.6 km) wide with wind speeds over 200 mph (320 km/h).
"These were the most intense super-cell thunderstorms that I think anybody who was out there forecasting has ever seen," Greg Carbin of the Storm Prediction Center in Oklahoma told the Associated Press (AP) news agency.
In Alabama, as many as one million people were without power on Thursday, as emergency workers and 2,000 soldiers scoured the wreckage for survivors.
"We can't control when or where a terrible storm may strike, but we can control how we respond to it," Mr Obama said.
Many people were picking through the remains of destroyed homes.
Alabama Governor Robert Bentley said he expected the death toll to rise as more bodies were discovered.
The number of deaths from the recent series of tornadoes is the greatest in the US since 1974, when an sequence of twisters claimed 315 lives in 13 US states.
Devastation in Tuscaloosa
One of the worst-hit towns was Tuscaloosa, which was struck by a massive tornado some experts say could have been an EF5, the strongest category of tornado.
Mayor Walter Maddox said after an aerial tour that it had torn a streak of "utter destruction".
"We have neighbourhoods that have been basically removed from the map," he said.
The city's emergency building was one of those destroyed, AP reported.
One Tuscaloosa resident, Angela Smith, whose neighbour was killed, told Reuters: "I made it. I got in a closet, put a pillow over my face and held on for dear life because it started sucking me up."
Another town, Hackleburg, was reported to be "90% destroyed".
The mayor of Birmingham, William Bell said "whole neighbourhoods of housing, just completely gone. Churches, gone. Businesses, gone... [it] seems like a bomb has been dropped".
More than 25 people died in Phil Campbell, a town of about 1,000 in north-west Alabama.
Jerry Mays, the town's mayor, said the tornado that destroyed the town's grocery store and medical clinic was a half-mile wide and travelled for about 20 miles (32 km).
"We've lost everything. Let's just say it like it is," Mr Mays said.
"I'm afraid we might have some suicides because of this," he added.
The storms forced the Tennessee Valley Authority to close three nuclear reactors at a power plant in Alabama. Hundreds of thousands of homes have lost power as a result.
Mississippi reported 33 deaths on Tuesday and Wednesday. In Smithville, Mississippi, many buildings were ripped open, including a church, the city hall and the post office.
At least 14 people have been killed in Georgia and five in Virginia.